A major factor adversely affecting the indoor performance of a radio-frequency (RF) wireless local-area network (WLAN) is multipath fading, where the signal reaching the antenna of a receiving modem is subject to distortion due to the superimposition of multiple versions of the transmitted signal propagated over different paths. One manifestation of this distortion is that signals arriving by different paths are liable to cancel each other out, thereby attenuating the received signal to such a degree that the receiver can no longer reliably reconstruct the transmitted data. WLANs are designed to allow any of the transmitters and receivers to be moved within a certain range. However, multipath fading renders the received signal strength liable to appreciable fluctuation as the transmitter and/or receiver are moved from one location to another. Due to the relatively high radio frequencies used by RF WLANs, the received signal strength is liable to vary considerably over even a small distance. In order to mitigate the attenuative effect of multipath fading, an RF WLAN receiver commonly employs the technique of space-diversity reception, where the receiver compares the strength and/or signal-to-noise ratio of the signals received by a plurality of physically-separated receiving antennae, and selects the best signal.
ANSI/IEEE standard 802.11 “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”, approved Mar. 19, 1999, specifies a data communications protocol for use at speeds of up to 2 million bits per second (Mbit/s) that enjoys great popularity in the WLAN marketplace. Several supplements and amendments to this standard have since been published. In particular, 802.11b, approved Sep. 16, 1999, extends the maximum transmission rate to 11 Mbit/s, and 802.11g, approved Jun. 27, 2003, specifies higher transmission speeds of up to 54 Mbit/s. The 802.11b standard specifies the use of the Frequency-Hopping Spread Spectrum (FHSS), Direct-Sequence Spread Spectrum (DSSS), and Complementary Code Keying (CCK) modulation modes for RF communications in the 2.4 gigahertz (GHz) Industrial, Scientific and Medical (ISM) band. Standard 802.11g also uses the Orthogonal Frequency Division Multiplex (OFDM) modulation mode. The 802.11 standard in combination with its supplements and amendments is hereinafter referred to as 802.11.
Standard 802.11 defines a physical-layer, data-transfer protocol called the Physical Layer Convergence Protocol (PLCP), in which data are encapsulated in frames known as PLCP Protocol Data Units (PPDUs). The standard suggests the use of the preamble field, which is the first transmitted portion of PPDUs, as an appropriate interval for antenna selection in a space-diversity receiver. For the FHSS mode, subclause 14.3.2.1.1 of the standard defines a preamble field synchronization (SYNC) subfield consisting of 80 bits of alternating ones and zeroes, lasting on the order of 80 microseconds, and suggests the use thereof as a basis for antenna selection in a receiver equipped with space-diversity reception. For the DSSS mode, subclause 15.2.3.1 defines the corresponding preamble field SYNC subfield that consists of 128 bits of scrambled ones, lasting on the order of 128 microseconds. For the OFDM mode, subclause 17.3.3 of 802.11a specifies the duration of the PLCP preamble field, which is on the order of 16 microseconds.
IEEE standard 802.11 WLAN receivers commonly perform comparative signal strength and signal-to-noise ratio measurements for each of the antennae during transmission of the preamble field. However, the relatively short duration of the preamble field in the 802.11g OFDM mode renders these measurements much less reliable, and receivers often fail to select the optimum antenna.